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Old 10-11-2010, 09:49 AM   #1
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Soot Inside Gas Furnace

My home was a foreclosure so I have no idea when its furnace was last used. It's a gas furnace that is older although I don't know how old. My HVAC technician discovered some soot that is isolated to the inside of the furnace near the flame. It's not mounds of soot but more like a dusting of soot. There's no soot on the walls and floors near the furnace. The HVAC tech used a camera to check the heat exchanger and didn't see any cracks or an obvious reason for the soot.

I am trying not to replace my furnace if at all possible. The HVAC tech has some concerns given that he's not sure where the soot is coming from and the heat exchanger appears to be fine. My plan of action is to have the furnace cleaned thoroughly, get it running, and test for carbon monoxide before deciding that it needs to be replaced.

In this case, would any of the HVAC pros agree with my course of action or would you typically recommend to your clients that the furnace be immediately replaced? Does anyone have any thoughts on where the soot may be coming from so I can have a more educated conversation with my HVAC tech?

Thanks in advance.


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Old 10-11-2010, 11:48 AM   #2
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Did the tech ever fire up the unit to watch how the burning of the flame was, or check exhaust duct work? If all they did was give a once over on the equipment without running it, you will never now.


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Old 10-11-2010, 01:12 PM   #3
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Does it have a pilot light/flame? If so they can burn dirty and produce soot which is not good but not necessarily a huge problem. Get it professionally cleaned and inspected by another tech with more experience. Try get them to check the CO ppm reading in the exhaust to see if it is excessive. Post the make and model # and serial # here and we can give you a better idea if it is a known hazardous unit.
"Cut it twice and it is still too short".
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Old 10-11-2010, 04:00 PM   #4
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Thank you for your replies. The tech cleaned the furnace and lit the flame which burned blue. He also looked into the chimney and duct work. He mentioned that the duct work connected directly to the furnace was not pulling air out. To test this he lit a match next to the duct work and noted that the flame wasn't being pulled towards & out of the duct work.

I will discuss the CO test as suggested. I know the furnace is made by RUUD although I don't have the model and serial number. I'll post that when I get it.

Thanks again.
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Old 10-11-2010, 05:02 PM   #5
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Soot is from incomplete combustion. Can be caused by improper set up. or a negative draft in the house caused by some other source.

The furnace should be tested with a combustion analyzer. And then its combustion set up properly.

Dirty furnaces can be cleaned. And then set up properly. And if the source or cause of the poor combustion was from an exhaust fan somewhere else in the house. You need to correct that. instead of replacing the furnace. And still having the same problem.
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